La Niña brings soup to the Bay

It’s been a long time between drinks, but La Niña is back in town! La Niña events (the flipside of drier El Niño events) bring above average rainfall to eastern and northern Australia, due to cooler sea surface temperatures in the central and eastern tropical Pacific ocean.

The resulting increased flows from the Yarra River are a major source of nutrients entering the Bay. These nutrients trigger growth of micro-algae which are eaten by water bugs, eaten by fish, eaten by larger fish, which are eaten by even larger predators including people.

Brown and swollen Yarra at Studley Park

Nutrients and small soil particles washed from the Yarra catchment give the river its typical brown colour. The resultant algal growth in the Bay can be soupy green as recently seen from St Kilda Pier.

Green water at St Kilda

La Niña events can last less or longer than a year, but usually arrive in autumn, peak in spring before fading in late summer to end in autumn. The Bay is geared to cope with the seasonal higher nutrient level, but when combined with extra nutrients released from sediments by dredging, this may cause an algae overload.